During the commercial production of paper, a uniform and continuous stream of a dilute water suspension comprising pulp fibers and other additives are allowed to flow readily through a narrow slot onto a traveling wire mesh. Most of the water drains rapidly through the wire mesh as it moves over a series of table rolls and suction boxes which support the wire and remove water from the forming sheet. As more water is drawn from the wet sheet, the consistency of the sheet gradually increases to about 20% at which point the paper sheet is strong enough to be transferred to a porous fabric blanket where it is pressed and looses water. Most of the remaining water, more than half the dry weight of the sheet, is evaporated out of the paper leaving about 5% to 8% moisture in the paper to minimize dimensional changes which occur as the paper establishes equilibrium with the relative humidity of the air in the production plant.
The addition of various chemicals to improve the properties of paper have been described in the prior art. For example, U.S. Pat. No. 4,954,220 to Rushmere relates to the use of anionic polysilicate microgels in combination with an organic polymer to flocculate pulp and papers fines in a papermaking operation. The polysilicate microgels are formed by the partial gelation of an alkali metal silicate or a polysilicate such as sodium polysilicate. The microgels are referred to as active silica in contrast to commercial colloidal silica and consist of aggregates of very small particles, e.g., 1 nanometer, and form three-dimensional networks. The microgels are described as having very high surface areas and are used in the papermaking process in conjunction with cationic polymers. In the section of the patent under "Detailed Description of the Invention", reference is made to the description of active silicas in the book "The Chemistry of Silica" published in 1979 by R. K. Iler as providing a disclosure of active silica microgels which can be used in the patented invention. The patent then sets forth a series of alternative methods by which polysilicate microgels may be prepared, including: 1) aqueous solutions of alkali metal silicates which have been acidified by acid exchange resins; 2) inorganic and organic acid aqueous solutions of alkali metal silicates which have been acidified by acids, salts and gases; 3) aqueous solutions of alkali metal silicates to which alkali metal salts of amphoteric metal acids have been added; and 4) alkali metal silicates to which certain organic compounds have been added. The patent then concludes in column 3, lines 50-55 that the simplest and most economic methods for the preparation of polysilicate microgels are products produced by acidification of sodium polysilicate solutions with a common mineral acid or addition of a gel initiator such as alum, sodium borate or sodium aluminate to a sodium polysilicate solution.
EKA Nobel PCT Published Application WO-90/107543 discloses a process for the production of paper in which three components are added to the paper suspension, a cationic starch, a cationic polyacrylamide, and a polymeric silicic acid, to improve retention and water removal in paper production. This PCT application discloses that the polymeric silicic acid is the same product as disclosed in Swedish Patent Application No. 8801951-8, which product has a very high specific surface area in the range of 1100 to 1700 m.sup.2 /g and can be prepared by acidification of an alkali metal silicate such as sodium waterglass, the acidification being conducted by use of a mineral acid such as sulfuric acid or acid ion exchangers. In Example 1 on page 7, the polymeric silicic acid was prepared from waterglass which was diluted with water to an SiO.sub.2 content of 5% by weight, after which the aqueous solution was ion exchanged with ion exchange resin Amberlite IR-120 to a pH of 2.3.
EKA European Patent Application No. 041056, directed to a paper making process, is disclosed in column 1 of the EKA Nobel published PCT application. This published application discloses a papermaking process which uses a binder of colloidal silicic acid and cationic starch added to the stock for improving the paper or retention of the stock components. Colloidal silicic acid is described, for example at page 7, as being a known material made by reacting waterglass with sulfuric acid by known procedures to provide a silica having molecular weights up to about 100,000. However, the patent states that this product is unstable and then points out at page 8 that by the invention disclosed in the European Patent Application, superior results are obtained through use with the cationic starch, of a colloidal silicic acid in the form of a sol, wherein the colloidal silicic acid in the sol will have a surface area of from 50-1000 m.sup.2 /g and preferably 200-1000 m.sup.2 /g. The publication states that such silicic acid sols are available from various sources including Nalco, DuPont and EKA.
At page 20, Example IV, there is a specific description for producing a colloidal silicic acid component wherein 100 ml of waterglass were diluted with 160 milliliters of water and slowly fed into 130 milliliters of 10% sulfuric acid under vigorous agitation. When all the waterglass had been added, the pH was 2.7 and the SiO.sub.2 content was 8% by weight. This acid sol was diluted to 2% SiO.sub.2 by weight, added to an English China Clay grade C followed by addition of 2% cationic starch solution. These mixtures were then added to the paper.
Finnish Application No. 68283 discloses a two-component system used in the paper making process. The system comprises a cationic starch and a colloidal silica sol. This system is intended for improving retention and strength properties of the paper product. However, this two-component system is not commercially attractive due to the high price of the colloidal silica sol.
Finnish Application No. 894362 proposes a two component system in which colloidal silica sol is replaced with a certain anionic polysilicate microgel, which is prepared by a partial gelation of an alkali metal silicate by adding certain initiators to an aqueous solution of the alkali metal silicate.
None of these prior publications recognize that a temperature-activated polysilicic acid prepared by heating an acidified aqueous alkali metal silicate has improved properties as a retention agent in paper production. Further, none of these publications disclose a process for the production of paper utilizing a temperature-activated silicate for improvement of retention and water removal.
The present invention provides a novel temperature-activated polysilicic acid prepared by heating an acidified aqueous alkali metal silicate. Use of the novel temperature-activated silicate in the paper production process represents a significant improvement over the prior art for the following reasons: 1. They improve the mechanical properties of the finished paper; 2. They improve the recovery of fines and colloids from the water cycle; 3. They improve filterability; 4. They improve grammage and ash retention, i.e., filler retention; and 5. They are economical and easy to use.